After working my new to me 3600 for a couple hours yesterday I discovered the oil cooler is leaking. It's the cooler itself, looks like it had been hit sometime in it's life and finally gave up. A quick on-line search reviled that oil cooler must be made out of some kind of gold alloy. Any reason not to just use a ~$30.00 transmission cooler from the local auto parts store?
Slowly getting the quirks worked out on this tractor, also discovered a bad wheel bearing on the right front and it has way too much toe-in.